Millions of sufferers in southern England were affected before it spread northwards.

The Met Office has teamed up with the NHS to identify which of the 150 different grass pollen cause the most allergies, with research finding more than half of sufferers (57%) do not know what is causing their symptoms.

Image: A map shows where pollen count is low, medium, high, and very high on Thursday

A poll of 2,000 British hay fever sufferers also revealed two-fifths (41%) of them experience the condition so badly it ruins their summer.

Experts have said many people may be causing their symptoms to worsen unnecessarily.

More than a third of respondents (35%) said they frequently hang washing out to dry in the summer, seemingly unaware that pollen sticks to their clothes.

A further 40% said they leave their windows open to cool their home in the summer, allowing pollen inside.

And only 49% of sufferers said they take antihistamines to relieve their hay fever.

The Met Office's Yolanda Clewlow, manager of the UK pollen network, said: "We know how seriously hay fever can impact people's lives in the UK, particularly as a result of grass pollen.

"This has led to our involvement in a dedicated research programme to identify the most significant of the 150 different species of grass pollen in the UK.